Exhaust Driven (Blown) Diffusers
Rear downforce is very important for driver confidence. If the driver feels good rear end stability he will push harder, so the gain on the stopwatch from this kind of development is often not what a simulator tells you it will be, but what the driver actually delivers from it.
A diffuser is actually a simple device. A diverging and expanding duct creates low pressure area under the car, creating negative lift, i.e. downforce. More about diffuser itself, you can read in my article hire.
A blown diffuser is basically a way of using the exhaust gases to interact with the diffuser airflow. There are two main purposes for this;
1.to try to move the wake from the rear wheels outwards where it will cause less disturbance
2.To re-energize the low pressure air at the back of the diffuser to create more rear downforce.
Exhaust blown diffuser on Renault RE40 (V6 Turbo) F1
The blowing effect of exhausts is not a new thing, and has been a factor in Formula 1 since 1983 when Renault first routed their exhausts into the diffuser aft of the flat bottom on the RE40 (V6 Turbo). Back then the diffuser was 1000 millimeters width and not split like the current versions are (due to the step plane regulations). Before this everyone had routed the exhausts into the area of least influence, usually above the gearbox or with long pipes through the rear suspension. This created no downforce benefits.
Blown diffuser on MARCH-911b Formula 1 car.
Picture above showing engine bay and exhaust pipes entering diffuser. On picture right, exhaust pipes disappearing.
The effect of the exhaust gases speeding up air flow through the diffuser, was improving venturi effect under the car, energizing the flow and thick boundary layer, and with in this way powering the diffuser will gain more downforce without drag penalty.
The downside was the almost an ON/OFF effect (throttle position and quantity and speed of exhaust gases) of the exhaust blowing during the course of the lap varying the downforce effect. Idea of blowing into diffuser was excellent but was not very successful. Problem was that exhaust gases don't have constant speed. More driver press the throttle pedal, higher the speed and quantity of the exhaust gasses and better efficiency of diffuser. Less throttle, lower the exhaust gasses speed. With lower speed of gasses, diffuser efficiency is lower and driving with lower downforce on the back of the car is not so pleasant thing to do. The worst part of the story is that during the cornering, when you need more downforce, drivers usually need to lift a foot a bit, losing downforce where they need it more. Read later more how this effecting RedBull RB06 and 07.
Turbo-charged engines are not critical to exhaust tuning (pipe length and position), and ones the exhaust gases have passed through the turbo charger they have much constant speed and slowing lag. It was during the turbo era that aerodynamicists discovered that using the exhaust gas flow to blow the diffuser of the flat-bottomed cars, increased the air flow extraction under the car and so increase downforce. The move away from turbos and their effect of smoothing the flow out of the exhausts made this a bigger problem. As we sad before, when the driver lifted the throttle, the flow reduced and the downforce reduced. From this time on, exhaust system design has been a compromise between aerodynamic needs and power, and the two departments (aero and engine) have strenuously researched the affects in order to make their individual cases as priority.
When diffuser blowing first came in, the exhaust was arranged to blow tangentially along the surface of the inclined diffuser. The high velocity gases entrained air, energizing the thick boundary layer, and effectively powered the diffuser as it drew air under the car. The fact that the throttles controlled the gas flow would appear to contravene the regulation that prohibits any part of the car that affects aerodynamic performance from being moveable. However, wherever the exhaust exited it would affect the aerodynamics, so diffuser blowing has escaped the regulation on the basis that if it was banned, all exhausts would be illegal.
During the construction of an F1 car, wind tunnel models of the Formula 1 car incorporate small, ejector-type, air driven pumps that simulate the air flow into the ram intake and doubled it as the exhaust flow. The effect of throttle-open and -closed can be tested and the changes in downforce and, more importantly the centre of pressure of the car can be measured. In recent years, the effect of the exhaust on the centre of pressure, and its variation with throttle opening have led to the true blown diffuser being abandoned, and exhausts exits moved to blowing over the top of the lower body rear edge. In this position, they probably did more to increase the radiator air exit flow than influence the underbody flow.
However, two further trends have led to Ferrari considering and then adopting an alternative exhaust arrangement. In the quest to move weight forward that has resulted from the width limitation on the rear tires and the grooves in the treads which led to Bridgestone introducing a wider front tire in 1998, the engine has moved forward relative to the rear of the bodywork, defined in the regulations by the rear axle centre line.
At the same time the peak RPM of engines climbed relentlessly upward, now 18,000rpm. Thus, while exhaust pipes needed to become shorter to stay in tune with the higher RPM (higher RPM-s - shorter pipes, Lover RPM-s - longer the pipes), the exhaust pipes had to be longer to reach the trailing edge of the underbody. For Ferrari, the arrangement that provides for nearer optimum length exhaust pipes, by leading them the short distance from the engine to an exit port set into the top surface of the bodywork (chimney), is better than one that still blows into the base region around the trailing edge of the diffuser.
Blown diffuser on McLaren MP4/17. Not in the exactly in the plain view, but you have idea
In this time, Adrian Newey's McLaren MP4-17 blows the exhaust into the centre of the diffuser to improve the flow along the step lane and under the plank. Problem was very strong ON/OFF effect produced during throttle lift.
Besides this two, there are not many other areas that exhausts could route to gain benefit. You need to route the exhausts around the rear of the sidepods somewhere, so there is a limit to the flows that can be affected, plus they cannot route beyond the rear edges of the diffuser (it's in the regulations). Exhaust gases can be used for speeding flow up where it is low energy or bad quality (along the step plane, McLaren), or for further speeding up flow over and aerofoil (rear wing/flip ups, Ferrari).
The FIA has acted several times since the mid eighties to cap the potential of the diffuser by reducing its length, height, ride height and position relative to the rear axle. Moving air through the diffuser is the key to it producing downforce, or Mass flow as the aerodynamicists call it.
Onset airflow is another factor controlled by the front wing, bargeboards and the floor itself, but this is somewhat capped by what can be achieved with the limited devices the rules allow for.
Then there is the flow over the top of the diffuser, this has been perhaps the biggest area of development in recent years. By ending the diffuser with a gurney flap, the airflow over the top of the diffuser can actually aid airflow extraction underneath the diffuser. This is the reason sidepods have become slimmer, undercut and the diffuser appears more exposed amongst the coke bottle bodywork. Effectively the harder the air flows over the diffuser, the more powerful the gurney can be in puling airflow from inside the diffuser; this makes the diffuser act as though the exit is larger and makes more downforce.
With 2009 downforce reduction rules, the diffuser continues to be the dominant factor in aero design. Making the most of creating low pressure under the rear of the cars bodywork is as important as ever. Last year (2009) we saw teams exploit rule loopholes to create additional underbody air inlets feeding and energizing larger exit areas, known as the double diffuser. This year teams have further exploited these rules for ever larger inlets and outlets.
However it has again fallen to Red Bulls Adrian Newey to look at the history book and re-invent a concept that has since fallen out of favor. Last year he did the same with reinvention of the pull rod rear suspension and this year it has been the exhaust driven diffuser. By mounting the exhaust outlets in line with the floor, they blow through and over the diffuser driving greater airflow and hence creating more downforce.
Red Bull surprised everyone with their revised car that appeared on the last day of 2010 preseason testing. The RB6 car sported revised exhaust systems with a low exits. Although it was first tested with the conventional RB5 exhausts, it was only at the last test day the team unveiled the secret exhaust development. Even replacing the old exhausts with look-a-like stickers to fool the unwary.
The RB5 that preceded 2010 year's RB6 car, already had high placed rear wishbones, and this allowed the subsequent car to run exhausts mounted low down and exit well below the wishbone, avoiding any overheating issues of the carbon fiber components. Teams have run exhausts in very close proximity to the wishbones now for many years, the differing strategies teams employ reduce the thermal load on the carbon fiber wishbones. Either gold foil film, extra carbon fiber heat shield or these are often coated with ceramic finishes to reflect heat. (Check on the heat shilding article here).
Contrary to the popular belief, the low exhaust position is not related to the Red Bulls Pull rod suspension; in some respects having the exhaust in close proximity to the pull rod/rocker linkage is undesirable. But the exhaust positioning is probably more sensitive to wishbone position, so much that teams aiming for low wishbones may have problems packaging the exhaust under the suspension. McLaren and Virgin have notably low wishbones.
However, the Newey designed solution on the RB6 is a little more complicated than it first appeared. In the RB6's case Newey made a vertical window in the diffuser to allow the diffuser to be blown both under and over by the exhaust. This helps the airflow going up the outside shoulder of the upper diffuser deck, which probably has little energy and struggles to keep attached, and the high speed exhaust gas will drive more flow through the diffuser to increase downforce.
Criticism aimed before at exhaust driven diffuser is their sensitivity to the throttle. As we see before, this issue was mainly related to when the exhausts were placed right on the kickline between the floor and diffuser, thus ON/OFF effect on downforce was much more pronounced. Plus, the placement of the exhaust exits some way upstream of the diffuser should allow a better compromise between downforce and sensitivity. Newey knows a thing or two about blown diffusers, the McLarens retained diffuser exhausts exits all the way to the MP4-17, even then the switch to periscope (chimney) exhausts was largely driven by other engine packaging factors. Even the still born MP4-18 aimed to have diffuser exiting exhausts.
Red Bull pioneered the technology and has really maximized it.
One of Red Bull's secrets is a setting and mapping on the Renault engine for use on the final crucial lap in qualifying. Engine ignition is retarded by as much as 35-40% on the over-run (in other words when the driver lifts off the throttle), which maintains exhaust gas pressure even when the driver lifts off the throttle. By retarding the ignition when the driver lifts off, the fuel is no longer burnt inside a closed combustion chamber, but instead the fuel and air burn in the exhaust pipe, the expandign gasses blow out of the exhaust exit as though the engine is running . This creates a more constant flow of exhaust gasses between on and off throttle. This maintains the performance of the blown diffuser and keeps the downforce up when it's most needed. In this way they managed to avoid the main problem of an exhaust blown diffuser whereby when a driver lifts off the throttle for a corner, the downforce goes missing when you most need it and the rear stability changes.
It's not something you can do for more than a lap or two as the engine temperatures go sky high, which damages the engine, but it gives that vital fraction of a second which keeps Red Bull ahead of the rest in qualifying. The problem here is that the engine mapping uses more fuel and creates excessive heat in the exhaust pipes and at the exhaust valve. Renault reported that both Red Bull and Renault used 10% more fuel in Melbourne compared to last year, most likely due to these off-throttle mappings.
Another misconception of the low exhaust is the effect on tire temperature. It's possible that the exhaust does affect the inner shoulder of the rear tires, but this may well be an effect teams want to discourage. Any complete tire heating will certainly be secondary benefit of the system and the sole reason for going with low exhausts. It's interesting to note Red Bull have run a fence on the floor between the exhaust and rear tire. This probably helps keep unwanted heat from the tires. But in Canada, where tire temperatures were low, this fence was removed. It could be that the tire heating effect could be a tunable parameter, by varying the heat shielding around the coke bottle area.
Of course, other teams want to dissolve RedBull advantage, and start to develop their own blown diffusers. So far we have seen Ferrari, Renault and Mercedes have followed Red Bulls 'back to the future' exhaust/diffuser solution on 2010 European GP in Valencia. McLaren and Williams are expected to follow for the next race At Silverstone. Ferrari, Renault and Mercedes start the race with a diffuser blown only over the top, without window, which perhaps offers less potential then a through blown diffuser, but at least will be legal next year when double diffuser is banned by new rules, and preventing openings in the diffuser. The blown element operates independently of the "double" element of the diffuser and whereas double diffusers are banned from next season, the blown diffuser is here to stay.
On the end of 2010 season, Ferrari perfected their own blown diffuser. In the place of vertical windows used by RedBull racing RB6 racer, Ferrari employed horizontal slots over the top of diffuser.
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The neatly-packaged Red Bull exhaust exit in the begining of 2011 |
A blown diffuser increases downforce on all corners, with the greatest effect on medium to low speed corners. That's the reason why is Red Bull so strong on the tracks with high speed corners, and less on the tracks like Monaco, Montreal Hungaroring or Valencia.
BTW, Red Bull Racing clinched both the constructors' championship and the drivers' crown with Sebastian Vettel, with the victory in Abu Dhabi, the last race of 2010 season and Vetel become Formula 1's youngest-ever world champion, only 23 years old. This is all the proof that is needed about how good the outfit is now, with Adrian Newey as technical director and main designer.
